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Carbon (C) is an important isovalent impurity in silicon (Si) that is inadvertently added in the lattice during growth. Germanium (Ge), tin (Sn), and lead (Pb) are isovalent atoms that are added in Si to improve its radiation hardness, which is important for microelectronics in space or radiation environments and near reactors or medical devices. In this work, we have employed density functional theory (DFT) calculations to study the structure and energetics of carbon substitutional-isovalent dopant substitutional C_sD_s (i.e., C_sGe_s, C_sSn_s and C_sPb_s) and carbon interstitial-isovalent dopant substitutional C_iD_s (i.e., C_iGe_s, C_iSn_s and C_iPb_s) defect pairs in Si. All these defect pairs are predicted to be bound with the larger isovalent atoms, forming stronger pairs with the carbon atoms. It is calculated that the larger the dopant, the more stable the defect pair, whereas the C_sD_s defects are more bound than the C_iD_s defects.